Though clinically safe during the immediate operative period, little is known of the long-term effects of commonly used myocardial protection techniques on structure and function of the hearts of patients undergoing open heart surgery. The proposed research will investigate the long-term effects of hypothermic and/or hyperkalemic ischemic arrest on myocardial structure, function and metabolism in the dog and rat. Local profound cardiac hypothermia as well as potassium arrest induced by intra-aortic injection of hyperkalemic solutions will be used to protect the dog and rat heart for periods of 60-180 minutes without coronary perfusion. In the dog, myocardial high-energy phosphate compounds will be measured by stop-freeze biopsies before, during and after periods of hypothermic and hyperkalemic ischemic arrest; cardiac hemodynamics will be measured preoperatively and four weeks postoperatively; and histopathologic examination and quantitation of fibrosis by light microscopy and planimetry will be determined four weeks postoperatively. A chronic non-working heart model in the rat will be utilized to evaluate prolonged periods of hypothermic and hyperkalemic ischemic arrest. This will be accomplished by heterotopic heart transplantation to the abdomen in isologous strains of Lewis rats. Following four weeks of implantation the hearts will be removed and examined histologically and the degree of fibrosis quantitated by planimetry. These experiments will provide a quantitative assessment of the long-term functional and histologic effects of the most commonly used myocardial protection techniques and may allow a more scientific approach to intraoperative myocardial protection.